Of great importance to man is the control of pathological cellular proliferation such as that which occurs in the case of cancer. Considerable research and resources have been devoted to oncology and antitumor measures including chemotherapy. While certain methods and chemical compositions have been developed which aid in inhibiting, remitting, or controlling the growth of, for example, tumors, new methods and antitumor chemical compositions are needed. Anti-proliferative agents can also be useful in treating autoimmune diseases and inflammatory disease.
In searching for new biologically active compounds, it has been found that some natural products and organisms are potential sources for chemical molecules having useful biological activity of great diversity. For example, the diterpene commonly known as paclitaxel, isolated from several species of yew trees, is a mitotic spindle poison that stabilizes microtubules and inhibits their depolymerization to free tubulin (Fuchs, D. A., R. K. Johnson [1978] Cancer Treat. Rep. 62:1219-1222; Schiff, P. B., J. Fant, S. B. Horwitz [1979] Nature (London) 22:665-667). Paclitaxel is also known to have antitumor activity and has undergone a number of clinical trials which have shown it to be effective in the treatment of a wide range of cancers (Rowinski, E. K. R. C. Donehower [1995] N. Engl. J. Med. 332:1004-1014). See also, e.g., U.S. Pat. Nos. 5,157,049; 4,960,790; and 4,206,221.
Marine sponges have also proven to be a source of biologically active chemical molecules. A number of publications disclose organic compounds derived from marine sponges including Scheuer, P. J. (ed.) Marine Natural Products, Chemical and Biological Perspectives, Academic Press, New York, 1978-1983, Vol. I-V; Uemura, D., K. Takahashi, T. Yamamoto, C. Katayama, J. Tanaka, Y. Okumura, Y. Hirata (1985) J. Am. Chem. Soc. 107:4796-4798; Minale, L. et al. (1976) Fortschr. Chem. org. Naturst. 33:1-72 Faulkner, D. J., Nat. Prod. Reports 1984, 1, 251-551; ibid. 1987, 4, 539; ibid 1990, 7, 269; ibid 1993, 10, 497; ibid 1994, 11, 355; ibid 1995, 12, 22; ibid 1998, 15:113-58; ibid 2000 17:1-6; ibid 2000 17: 7-55; ibid 2001, 18: 1-49; 2002, 19: 1-48; Gunasekera, S. P., M. Gunasekera, R. E. Longley and G. K. Schulte (1990) J. Org. Chem., 55:4912-4915; Horton, P. A., F. E. Koehn, R. E. Longley, and O. J. McConnell, (1994) J. Am. Chem. Soc. 116: 6015-6016.
The success of chemotherapy for the treatment of various cancers can be substantially negated though cellular mechanisms which have evolved to enable neoplastic cells to subvert the cytotoxic effects of the drug. Some cells have developed mechanism that confer resistance to a number of structurally unrelated drugs. This multi-drug resistance (or MDR) phenomenon may arise through a number of different mechanisms. One of these involves the ability of a cell to reduce intracellular concentrations of a given drug through efflux from cytoplasm through and out the cell membrane by a series of unique ATP-dependent transporter proteins called-P-glycoproteins (Pgp) (Casazza, A. M. and C. R. Fairchild [1996] “Paclitaxel (Taxol®): mechanisms of resistance” Cancer Treat Res. 87:149-171). The surface membrane, 170 kDa Pgp, is encoded by the mdr-1 gene and appears to require substrate binding before transport begins. A wide range of compounds. including a number of structurally unrelated chemotherapeutic agents (adriamycin, vinblastine, colchicine, etoposide and Taxol), are capable of being transported by Pgp and render the cell resistant to the cytotoxic effects of these compounds.
While many normal cell types possess Pgp, in general, tumor cell lines, which possess high levels of mRNA specific for Pgp, also exhibit overexpression of membrane Pgp and demonstrate resistance to various drugs. This intrinsic resistance can be increased multifold by incubation of cells with stepwise increasing doses of a particular drug over a period of several months. This can be further facilitated by the addition of the MDR reversal agent, verapamil (Casazza, A. M. and C. R. Fairchild [1996] supra) in combination with the particular drug. Drug resistant cell lines produced in this fashion exhibit resistance to drug cytotoxicity from 20 to 500 fold, compared to parental cell lines.
An additional target for cancer drug discovery is a high molecular weight membrane protein associated with multi-drug resistance properties of certain tumor cells known as the multidrug resistance-associated protein (MRP). MRP is a 190 kD membrane-bound glycoprotein (Bellamy, W. T. [1996], Annu. Rev. Pharmacol. Toxicol., 36: 161-183.) which belongs to the same family of proteins as the p-glycoprotein pump P-gp (Broxterman, H. J., Giaccone, G., and Lankelma, J. [1995], Current Opinion in Oncology, 7:532-540.) but shares less than 15% homology of amino acids with P-gp (Komorov, P. G., Shtil, A. A., Holian, O., Tee, L., Buckingham, L., Mechetner, E. B., Roninson, I. B., and Coon, J. S. [1998], Oncology Research, 10: 185-192.). MRP has been found to occur naturally in a number of normal tissues, including liver, adrenal, testis, and peripheral blood mononuclear cells (Krishan, A., Fitz, C. M., and Andritsch, I. [1997], Cytometry, 29: 279-285). MRP has also been identified in tissues of the lung, kidney, colon, thyroid, urinary bladder, stomach, spleen (Sugawara, I. [1998] The Cancer Journal, 8(2) and skeletal muscle (Kruh, G. D., Gaughan, K. T., Godwin, A., and Chan, A. [1995], Journal of the National Cancer Institute, 87(16): 1256-1258). High levels of MRP have been implicated in multidrug resistance (MDR) in cancers of the lung and pancreas (Miller, D. W., Fontain, M., Kolar, C., and Lawson, T. [1996]. Cancer Letters, 107: 301-306), and in neuroblastomas, leukemias and cancer of the thyroid (Kruh, G. D., Gaughan, K. T., Godwin, A., and Chan, A. [1995], Journal of the National Cancer Institute, 87(16): 1256-1258), as well as bladder, ovarian and breast cancers (Barrand, M., Bagrij, T., and Neo, S. [1997], General Pharmacology, 28(5): 639-645).
MRP-mediated MDR involves some of the same classes of compounds as those which are mediated by P-gp, including vinca alkaloids, epipodophyllotoxins, anthracyclins and actinomycin D (Barrand, M., Bagrij, T., and Neo, S. [1997], General Pharmacology, 28(5): 639-645). However, the substrate specificity has been demonstrated to differ from that of P-gp (Komorov, P. G., Shtil, A. A., Holian, O., Tee, L., Buckingham, L., Mechetner, E. B., Roninson, I. B., and Coon, J. S. [1998], Oncology Research, 10: 185-192). Drugs which would inhibit or which are not substrates for the MDR pump would, therefore, be useful as chemotherapeutic agents.